App Store app disappeared after iOS 9 update

First, look for the App Store app in all folders on your iPhone. If you still can't find it then swipe up from the middle of the screen to invoke Spotlight search, then type "App Store" and see if it finds it.

If not, then:

1) Hold down the sleep button and hte home button for several seconds until the iPhone screen goes black

2) Power the iPhone back on

3) Look for the App Store icon again.